This invention relates to a polyolefin masterbatch which can be used to prepare a polyolefin composition suitable for injection molding into relatively large articles. More particularly, the polyolefin composition can be injection molded into large objects which exhibit improved surface properties, particularly with respect to reduction of tiger striping.
Polypropylene and thermoplastic polyolefins have enjoyed wide commercial acceptance because of their outstanding cost/performance characteristics. For example, these polymers are used in molded-in color applications because of their good weatherability.
Polypropylene and thermoplastic polyolefins are generally injection molded into desired articles. Relatively large parts, such as automobile bumpers and fascia, offer particularly challenging problems such as cold flow and tiger striping. xe2x80x9cCold flowxe2x80x9d occurs when the molten polymer being injected into a mold begins to cool and solidify before the mold is completely filled with the polymer. xe2x80x9cTiger stripingxe2x80x9d refers to color and gloss variations on the surface of an injection molded article, which occur because of unstable mold filling properties of the molten polymer as it is being injected into the mold and formed into the desired shape.
The art has made various proposals to improve the physical characteristics of injection molded articles. Thus, Mizutani et al., xe2x80x9cFlow Mark Reduction of Metallic Colored PP,xe2x80x9d SAE 2000 World Congress (2000) discloses that the use of a low molecular weight polypropylene matrix having a broad molecular weight distribution improves the surface aesthetics of injection molded parts.
U.S. Pat. No. 5,055,528 discloses a sequential polymerization process for producing a propylene block copolymer having a melt flow rate of at least 10 g/min. and which is said to possess good flowability, moldability and impact strength.
U.S. Pat. No. 6,048,942 proposes a thermoplastic olefin composition useful for making molded articles having high surface gloss and mar resistance which contains (1) a propylene homopolymer, copolymer or terpolymer, (2) either a low molecular weight ethylene copolymer rubber, an elastomeric copolymer of ethylene and a C3-C8 xcex1-olefin made with a metallocene catalyst, or a mixture of the two, and (3) a lubricant.
Japanese Patent Publication No. 9-227,735 discloses a propylene-based resin composition said to be suitable for injection molding products having good appearance and difficult to notice flow marks. The composition contains specified amounts of a propylene-based copolymer and either an ethlene-based copolymer or a styrene-based block copolymer, optionally together with an inorganic filler and/or a nucleating agent.
An object of the present invention is to provide a polyolefin composition which can be injection molded into large articles which exhibit a good balance of physical properties and minimal tiger striping.
Another object of the present invention is to provide a convenient method for preparing this polyolefin composition based on a masterbatch composition.
A feature of the masterbatch composition of the present invention is a crystalline polypropylene component which has a bimodal molecular weight distribution. Another feature of the masterbatch composition is a xylene-soluble ethylene copolymer component having a high intrinsic viscosity of 4 to 9 dl/g at room temperature.
A feature of the polyolefin composition of the present invention is the presence of a crystalline polypropylene component which has a bimodal molecular weight distribution and at least 3 weight percent of a xylene-soluble ethylene copolymer component having a high intrinsic viscosity of 4 to 9 dl/g at room temperature.
An advantage of the masterbatch composition is its ability to be formulated with a broad molecular weight olefin matrix to form a polyolefin composition having high melt fluidity which can be injection molded into large articles such as automobile bumpers which exhibit a good balance of physical properties and reduced flow marks such as tiger striping.
In one aspect, the present invention relates to a masterbatch composition, containing (percent by weight):
A) 20%-90% of a crystalline polypropylene component containing from 25% to 75% of a fraction AI having a melt flow rate MFRI of from 0.5 to 10 g/10 min., and from 75% to 25% of a fraction AII having a melt flow rate MFRII such that a ratio MFRII/MFRI is from 30 to 2000; and wherein fractions AI and AII are independently selected from the group consisting of a propylene homopolymer, a random copolymer of propylene containing up to 8% of ethylene, and a random copolymer of propylene containing up to 8% of at least one C4-C10 xcex1-olefin; and
B) 10%-80% of a copolymer component of ethylene and at least one C3-C10 xcex1-olefin, the copolymer containing from 10 to 70% of ethylene, and optionally minor amounts of a diene, said copolymer being soluble in xylene at room temperature, and having an intrinsic viscosity [xcex7] of from 4 to 9 dl/g.
In another aspect, the present invention relates to a polyolefin composition suitable for injection molding, containing (all percentages by weight of the total composition):
A) from 0.1 to 50% of a masterbatch composition comprising
i) 20%-90% of a crystalline polypropylene component containing from 25% to 75% of a fraction AI having a melt flow rate MFRI of from 0.5 to 10 g/10 min., and from 75% to 25% of a fraction AII having a melt flow rate MFRI such that a ratio MFRII/MFRI is from 30 to 2000; and wherein fractions AI and AII are independently selected from the group consisting of a propylene homopolymer, a random copolymer of propylene containing up to 8% of ethylene, and a random copolymer of propylene containing up to 8% of at least one C4-C10 xcex1-olefin; and
ii) 10%-80% of a copolymer component of ethylene and at least one C3-C10 xcex1-olefin, the copolymer containing from 10 to 70% of ethylene, and optionally minor amounts of a diene, said copolymer being soluble in xylene at room temperature, and having an intrinsic viscosity [xcex7] of from 4 to 9 dl/g; and
B) from 50% to 99.1% of a crystalline propylene homopolymer having an isotactic index greater than 80 or a crystalline random copolymer of propylene and an olefin selected from the group consisting of ethylene and C4-C10 xcex1-olefins, provided that when the olefin is ethylene, the maximum polymerized ethylene content is 10% by weight, and when the olefin is a C4-C10 xcex1-olefin, the maximum content thereof is 20% by weight, the copolymer having an isotactic index greater than 85.
The masterbatch composition contains at least two components: a crystalline polypropylene component and a copolymer component. The crystalline polypropylene component is present in an amount ranging from 20 to 90%, preferably 30 to 80%, most preferably 50 to 80% of the total weight of the masterbatch composition. Conversely, the copolymer component is present in an amount of from 80 to 10%, preferably 70 to 20%, most preferably 50 to 20% of the total weight of the masterbatch composition, with the sum of the percentage amounts of the individual components of the masterbatch composition equal to 100 percent.
The fractions AI and AII which form the crystalline polypropylene component can each be a propylene homopolymer, a random copolymer of propylene containing up to 8%, preferably 0.2 to 5% of ethylene, or a random copolymer of propylene containing up to 8%, preferably 1 to 8%, of at least one C4-C10 xcex1-olefin which conforms to the formula CH2xe2x95x90CHR, wherein R is a linear or branched alkyl C1-8 radical or an aryl radical such as phenyl. Illustrative C4-C10 xcex1-olefins include 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene, with 1-butene being particularly preferred.
Fractions AI and AII differ from one another in their molecular weight distribution, as described by melt flow rate. Fraction AI has a relatively high molecular weight (low melt flow rate MFRI of from 0.5 to 10 g/10 min.) while fraction AII has a relatively low molecular weight (high melt flow rate). This relationship is defined by the ratio MFRII/MFRI, which can range from 30 to 2000, preferably 40 to 2000, more preferably 50 to 1000, still more preferably 100 to 800.
The inventors currently believe that a bimodal molecular weight distribution improves the spiral flow length, flexural modulus and surface aesthetics, including reduced tiger striping, of injection molded articles prepared from the polyolefin composition.
The melt flow rate of the masterbatch composition, as measured according to ASTM 1238, condition L (MFRL) can range from 0.1 to 50 g/10 min., more preferably from 0.3 to 20 g/10 min.
The manufacture of propylene homopolymers, random copolymers of propylene and ethylene and random copolymers of propylene containing up to 8% of at least one C4-C10 xcex1-olefin which are suitable for use as fractions AI and AII are well known to those of ordinary skill in the art. Generally, for propylene copolymers the content of polymer which is insoluble in xylene at room temperature (23xc2x0 C.) (i.e., substantially equivalent to the isotacticity index) for fractions AI and AII is not less than 80%, more preferably not less than 85%, and most preferably not less than 90% by weight. For propylene homopolymers, the content of polymer which is insoluble in xylene at room temperature is not less than 90%, more preferably not less than 95%, and most preferably not less than 97% by weight, based on the weight of the single fraction.
The other component of the masterbatch composition is a copolymer component containing from 10 to 70% of ethylene, at least one C3-C10 xcex1-olefin having the formula CH2xe2x95x90CHR where R is a linear or branched C1-8 alkyl radical or an aryl radical such as phenyl, and optionally a minor amount of a diene. The ethylene content of the copolymer component is preferably 15 to 60%, most preferably 15 to 50%.
Illustrative C3-C10 xcex1-olefins include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene and 1-octene, with propylene and 1-butene being particularly preferred.
Suitable dienes include butadiene, 1,4-hexadiene, 1,5-hexadiene and ethylidene-norbornene-1. When present, the diene is typically in an amount of 0.5 to 10% by weight with respect to the weight of the copolymer component.
The copolymer component is soluble in xylene at room temperature, and has an intrinsic viscosity [xcex7] at room temperature of from 4 to 9, preferably 5 to 8, most preferably 5.5 to 7 dl/g.
The inventors currently believe that a high intrinsic viscosity copolymer component also reduces the severity of tiger striping and results in an article having lower gloss.
The manufacture of ethylene copolymers suitable for use as the copolymer component is well known to those of ordinary skill in the art.
In a preferred embodiment, the masterbatch composition can be prepared by at least a three step sequential polymerization, in which components A) and B) are prepared in separate subsequent steps, operating in each step, except the first step, in the presence of the polymer formed and the catalyst used in the preceding step. The catalyst is added only in the first step; however, its activity is such that it is still active for all the subsequent steps. Thus, in at least two polymerization steps the relevant monomer(s) are polymerized to form fractions AI and AII and in the other step(s) a mixture of ethylene and the C3-C10 xcex1-olefin(s) is polymerized to form component B. Preferably, fraction AI is prepared before fraction AII and even more preferably, fraction B is prepared between fractions AI and AII.
The sequential polymerization is performed using a stereospecific Ziegler-Natta catalyst capable of producing polypropylene having an isotacticity index greater than 90%, preferably greater than 95%. The catalyst must also be sufficiently sensitive to molecular weight regulators (particularly hydrogen) to produce polypropylene having MFR values from less than 1 g/10 min. to 1000 g/10 min. or more.
Ziegler-Natta catalysts which possess these properties contain (i) a solid catalyst component comprising a titanium compound having at least one titanium-halogen bond, and an electron-donor compound, both supported on a magnesium halide in active form, (ii) a co-catalyst component comprising an organoaluminum compound, such as an aluminum alkyl compound, and optionally (iii) an external electron donor. Such catalysts are well known to those of ordinary skill in the art, as evidenced by U.S. Pat. Nos. 4,399,054, and 4,472,524, the disclosures of which are hereby incorporated by reference in their entirety.
The solid catalyst component of the Ziegler-Natta catalyst acts an internal electron donor, and may be a compound selected from the group consisting of ethers, ketones, lactones, compounds containing N, P and/or S atoms, and esters of mono- and dicarboxylic acids. Particularly suitable electron-donor compounds are phthalic acid esters, such as dilsobutyl, dioctyl, diphenyl and benzylbutyl phthalate. Other electron-donors particularly suitable are 1,3-diethers of the following formula: 
where RI and RII are the same or different and are C1-18 alkyl, C3-18 cycloalkyl or C7-C18 aryl radicals; RIII and RIV are the same or different and are C1-C4 alkyl radicals; or are the 1,3-diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6 or 7 carbon atoms and containing two or three unsaturations. Ethers of this type are described in published European patent applications 361493 and 728769. Representative examples of these diethers include 2-methyl-2-isopropyl-1,3-dimethoxypropane, 2,2-diisobutyl-1,3-dimethoxypropane, 2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane, 2-isopropyl-2-isoamyl-1, 3-dimethoxypropane, 9, 9-bis (methoxymethyl) fluorene.
The solid catalyst component may be prepared according to various methods. For example, a MgCl2. nROH adduct (in particular in the form of spheroidal particles) wherein n is generally from 1 to 3 and ROH is ethanol, butanol or isobutanol, may be reacted with an excess of TiCl4 containing the electron-donor compound. The reaction temperature is generally from 80 to 120xc2x0 C. The solid is then isolated and reacted once more with TiCl4, in the presence or absence of the electron-donor compound, after which it is separated and washed with aliquots of a hydrocarbon until all chlorine ions have disappeared.
In the solid catalyst component the titanium compound, expressed as Ti, is generally present in an amount from 0.5 to 10% by weight. The quantity of electron-donor compound which remains fixed on the solid catalyst component generally is 5 to 20% by moles with respect to the magnesium dihalide. The titanium compounds which can be used for the preparation of the solid catalyst component are titanium halides and titanium halogen alcoholates. Titanium tetrachloride is preferred.
The reactions described above result in the formation of a magnesium halide in active form. Other reactions are known in the literature, which cause the formation of magnesium halide in active form starting from magnesium compounds other than halides, such as magnesium carboxylates. The active form of magnesium halide in the solid catalyst component can be recognized by the fact that in the X-ray spectrum of the catalyst component the maximum intensity reflection appearing in the spectrum of the nonactivated magnesium halide (having a surface area smaller than 3 m2/g) is no longer present, but in its place there is a halo with the maximum intensity shifted with respect to the position of the maximum intensity reflection of the nonactivated magnesium dihalide, or by the fact that the maximum intensity reflection shows a width at half-peak at least 30% greater than the one of the maximum intensity reflection which appears in the spectrum of the nonactivated magnesium halide. The most active forms are those where the above mentioned halo appears in the X-ray spectrum of the solid catalyst component. Among magnesium halides, the magnesium chloride is preferred. In the case of the most active forms of magnesium chloride, the X-ray spectrum of the solid catalyst component shows a halo instead of the reflection which in the spectrum of the nonactivated chloride appears at 2.56 xc3x85.
The Al-alkyl compounds used as co-catalysts comprise the Al-trialkyls, such as Al-triethyl, Al-triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, or SO4 or SO3 groups. The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.
The electron-donor compounds that can be used as external donors include aromatic acid esters such as alkyl benzoates, and in particular silicon compounds containing at least one Sixe2x80x94OR bond, where R is a hydrocarbon radical. Examples of silicon compounds are (tert-butyl)2 Si (OCH3)2, (cyclohexyl) (methyl) Si (OCH3)2, (phenyl)2 Si (OCH3)2 and (cyclopentyl)2 Si (OCH3)2. 1,3-diethers having the formulae described above can also be used advantageously. If the internal donor is one of these diethers, the external donors can be omitted.
The molecular weight of the polymers may be regulated using known regulators, preferably hydrogen. By properly dosing the concentration of the molecular weight regulator in the relevant polymerization steps, the previously described MFR and [xcex7] values may be obtained. For preparation of fraction AI the hydrogen feed ratio H2/C3 (mol) may range from 0.0001 to 0.01. The hydrogen feed ratio H2/C3 (mol) for preparing fraction AII may range from 0.1 to 1.5, while the H2/C2 (mol) ratio for preparing copolymer component B may range from 0.0001 to 0.02.
The whole polymerization process, which can be continuous or batch, is performed according to known techniques and operating in liquid phase, optionally in the presence of an inert diluent, or in the gas phase, or by mixed liquid-gas techniques. It is preferred to carry out the polymerization in gas phase. Generally there is no need for intermediate steps except for the degassing of unreacted monomers. Reaction time, pressure and temperature relative to the two steps are not critical, however it is best if the temperature is from 20 to 100xc2x0 C. The pressure can be atmospheric or higher.
The catalyst can be pre-contacted with a small amount of olefin in a prepolymerization step using techniques and apparatus well known to those of ordinary skill in the art.
The masterbatch composition of the present invention can also contain additives commonly employed in the art, such as antioxidants, light stabilizers, heat stabilizers, nucleating agents, colorants and fillers.
The masterbatch composition of the present invention can be compounded with a propylene polymer such propylene homopolymers, random copolymers, and heterophasic copolymers to form a polyolefin composition. Accordingly, a second embodiment of the invention relates to a thermoplastic polyolefin composition suitable for injection molding, containing (all percentages by weight of the total composition):
A) from 0.1 to 50% of a masterbatch composition comprising
i) 20%-90% of a crystalline polypropylene component containing from 25% to 75% of a fraction AI having a melt flow rate MFRII of from 0.5 to 10 g/10 min., and from 75% to 25% of a fraction AII having a melt flow rate MFRII such that a ratio MFRII/MFRI is from 30 to 2000; and wherein fractions AI and AII are independently selected from the group consisting of a propylene homopolymer, a random copolymer of propylene containing up to 8% of ethylene, and a random copolymer of propylene containing up to 8% of at least one C4-C10 xcex1-olefin; and
ii) 10%-80% of a copolymer component of ethylene and at least one C3-C10 xcex1-olefin, the copolymer containing from 10 to 70% of ethylene, and optionally minor amounts of a diene, said copolymer being soluble in xylene at room temperature, and having an intrinsic viscosity [xcex7] of from 4 to 9 dl/g; and
B) from 50% to 99.1% of a crystalline propylene homopolymer having an isotactic index greater than 80 or a crystalline random copolymer of propylene and an olefin selected from the group consisting of ethylene and C4-C10 xcex1-olefins, provided that when the olefin is ethylene, the maximum polymerized ethylene content is 10% by weight, and when the olefin is a C4-C10 xcex1-olefin, the maximum content thereof is 20% by weight, the copolymer having an isotactic index greater than 85.
Importantly, the masterbatch composition should be present in an amount sufficient to ensure that at least 3 weight percent, based on total weight of the composition, of its copolymer component is present. The amount of masterbatch composition forming the polyolefin composition preferably ranges between 10 and 20 weight percent based on the total weight of the polyolefin composition.
The polyolefin composition may be manufactured by mixing the masterbatch composition and the crystalline propylene homopolymer or random copolymer together, extruding the mixture, and pelletizing the resulting composition using known techniques and apparatus.
The polyolefin composition may also contain a third component selected from the group consisting of
i) an elastomeric copolymer selected from the group consisting of (a) ethylene and propylene, (b) ethylene, propylene and a C4-C8 xcex1-olefin, and (c) ethylene and a C4-C10 xcex1-olefin, the copolymer optionally containing from about 0.5 to about 10% by weight of a diene, and containing less than about 70% of ethylene and being soluble in xylene at ambient temperature and having an intrinsic viscosity of about 1.5 to about 4.0 dl/g.;
ii) a heterophasic polyolefin composition comprising:
(a) about 30% to about 98% of a polymeric material selected from the group consisting of a propylene homopolymer having an isotactic index greater than 90, and a crystalline copolymer having an isotactic index greater than 85 of propylene and at least one xcex1-olefin of the formula CH2xe2x95x90CHR, where R is H or a C2-C6 alkyl group, the xcex1-olefin being less than 10% of the copolymer when R is H, and being less than 20% when R is a C4-C6 alkyl group or a combination thereof with Rxe2x95x90H, and
(b) about 2% to about 70% of an elastomeric copolymer of propylene and an xcex1-olefin of the formula CH2xe2x95x90CHR, where R is H or a C2-C8 alkyl group, the xcex1-olefin being about 45% to about 75% of the elastomeric copolymer, and about 10% to about 40% of the elastomeric copolymer being insoluble in xylene at ambient temperature, or an elastomeric copolymer of ethylene and a C4-C8 xcex1-olefin having an xcex1-olefin content of about 15% to about 60%;
iii) a propylene polymer material having a branching index less than 1 and strain hardening elongational viscosity; and
iv) styrenic elastomers such as styrene-ethylene-butene triblock copolymer (SEBS), hydrogenated SEBS and styrene-ethylene-propylene triblock copolymer (SEPS).
The polyolefin composition may also contain conventional additives such as mineral fillers, colorants and stabilizers. Mineral fillers which can be included in the composition include talc, CaCO3 and wollastonite (CaSiO3).
In an alternative embodiment, the polyolefin composition may be prepared without using a masterbatch. Instead, the components of the masterbatch may be individually prepared and mixed with the other components of the polyolefin composition, either simultaneously or in any desired sequence.
The polyolefin compositions of the present invention can be used to prepare finished or semi-finished articles having a desirable balance of properties, including flexural modulus, impact resistance and gloss. The polyolefin composition has particular utility in the production of injection molded articles because the resulting articles exhibit minimal tiger striping.